Effects of source of carbohydrate and protein and rumen-protected methionine on performance of cows.

Multiparous Holstein cows were fed diets consisting of alfalfa silage, corn silage, and a concentrate mixture containing primarily ground shelled corn or corn gluten feed; the diets were supplemented with 0 or 20 g/d of rumen-protected Met. The 183-d experimental period followed a 21-d covariate period beginning at calving. Data from early lactation (d 22 to 105) and the entire experiment were analyzed. Yields of milk and milk crude protein were not affected by treatment. Corn gluten feed increased the percentages of milk fat and total solids. The rumen-protected Met tended to increase both the crude protein and casein N content of milk. Body weight, milk fat percentage, and yields of milk fat, 3.5% fat-corrected milk, and total solids were greater when ground shelled corn and 20 g/d of rumen-protected Met or corn gluten feed and 0 g/d of rumen-protected Met were fed than when ground shelled corn and 0 g/d of rumen-protected Met or corn gluten feed and 20 g/d of rumen-protected Met were fed. A similar interaction for dry matter intake was significant only during d 22 to 204. Corn gluten feed increased plasma concentrations of His, Ile, Leu, and Val. Rumen-protected Met increased plasma concentrations of Met, decreased His, and tended to decrease Arg, Lys, and Orn. These data suggest that the dietary source of carbohydrate and protein can modulate the response of cows to rumen-protected Met.